1. Field of the Invention
The present invention relates to a method and apparatus for handling transmission time interval (TTI) bundling, and more particularly, to a method and apparatus for handling TTI bundling in a user equipment (UE) of a wireless communication system.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B (NB) alone rather than in NB and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In LTE, a technique of transmission time interval (TTI) bundling is introduced for improving uplink coverage according to the prior art. TTI bundling is performed by repeatedly coding and transmitting a same transport block in a set of consecutive subframes (e.g. TTI), and those repeatedly transmitted packets are named a bundle. UEs in cell boundary utilizing TTI bundling can reduce transmission delay and signaling of control channels, and enhance reliability and accuracy of transmission, such that LTE uplink coverage can be improved.
According to current specifications, TTI bundling is characterized as below:                (1) The same HARQ process is used for each packet of a bundle.        (2) TTI bundling is switched on/off per UE with higher layer signaling, e.g. Radio Resource Control (RRC) signaling. When switched on, TTI bundling would apply to all uplink transmissions using Physical Uplink Shared Channel (PUSCH).        (3) A bundle is treated as a single resource, i.e., a single grant and a single HARQ feedback (e.g. acknowledgement signal ACK or non-acknowledgement signal NACK) is used for each bundle.        (4) The retransmission of a bundle is also a bundle.        
It is worth noting that the timing relationship between the last transmission of a bundle and its HARQ feedback is the same as the normal HARQ operation (i.e. non-bundled operation), and that the HARQ Round Trip Time (RTT) for TTI bundling is twice the RTT of normal HARQ operation. That is to say, if the last transmission of a bundle occurs at TTI n, the HARQ feedback is received at TTI (n+4); and if a first transmission of a bundle occurs at TTI k, retransmission of the bundle starts at TTI (k+2*HARQ_RTT), where HARQ_RTT represents the round trip time of the normal HARQ operation. In addition, the size of a bundle, i.e. the number of TTIs required to transmit a bundle, is fixed as 4.
According to 3GPP meeting document R2-083726, the number of HARQ processes is reduced from 8 to 4 when TTI bundling is configured. In other words, the number of HARQ processes changes when the UE activates or deactivates the TTI bundling mode. As for detailed HARQ operations for TTI bundling, please refer to related meeting document R2-083724.
However, when TTI bundling is switched on (i.e. configured or activated), there is no special action specified in the prior art for the transport blocks (TBs) already stored in the uplink HARQ buffers. Based on the specification proposed in the meeting document R2-083724, these TBs may induce non-adaptive retransmissions in bundle way. Since uplink grants for these TBs were allocated before configuration of TTI bundling, retransmissions of these TBs should not be considered bundle retransmissions. Otherwise, it may cause interference to other UEs' transmissions. Besides, even the first non-adaptive retransmissions of these TBs (e.g. performed in a non-bundled way) after configuration of TTI bundling should not be generated because the timings of the corresponding HARQ processes have been shifted. Note that the number of HARQ processes is reduced to 4 after TTI bundling is configured, which causes the timing shifts. And, timing shift of an HARQ process means the allocated resources are used at a different timing from what have been scheduled, which may cause interference to other UEs' transmissions.
The similar situation occurs after TTI bundling is switched off (or deactivated) i.e. non-adaptive retransmissions of the old TBs may cause interference to other UEs' transmissions due to timing shifts of the HARQ processes.
On the other hands, the HARQ RTT for TTI bundling is doubled compared with the normal transmission operation. Please refer to FIG. 1, which is a schematic diagram of packet transmission and reception of a UE under the TTI bundling mode. Assuming that the length of TTI is T, and HARQ_RTT represents an HARQ RTT of the normal transmission operation with a fixed length 8T. Packets P1, P2, P3 and P4 are corresponding to a same transport block and transmitted at 4 consecutive TTIs from a UE, which means the packets P1, P2, P3 and P4 form a bundle BDL. When the UE finishes transmitting the bundle BDL at a time point 4T, the network end generates an HARQ feedback (i.e. acknowledgement signal ACK or non-acknowledgement signal NACK) between time points 7T and 8T to indicate the reception status of the bundle BDL. In this case, the packet P4 is not received successfully by the network end, so the network end transmits a non-acknowledgement signal NACK. When the UE receives the non-acknowledgement signal NACK from the network end between the time points 7T and 8T, the UE cannot retransmit the bundle BDL at a starting point of the next HARQ_RTT (i.e. at a time point 8T) since the processing time is too short. As a result, the UE starts to retransmit the bundle BDL after waiting for an HARQ RTT, namely at a time point 16T.
Simply speaking, the UE receives the HARQ feedback from the network end at the fourth TTI after transmitting the bundle BDL, and determines whether the bundle BDL needs to be retransmitted accordingly. If the bundle BDL needs to be retransmitted, the UE has to wait for an HARQ RTT to perform retransmission of the bundle BDL, i.e. transmit a retransmission bundle BDL_1. Thus, the HARQ RTT for TTI bundling is doubled compared with the normal transmission operation, which may cause transmission delay.